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MIT’s breakthrough self-powered sensor harvests energy from the air

For decades, the limitations of batteries have constrained how and where we monitor critical infrastructure. Imagine a scenario where sensors embedded within a ship's engine provide real-time data without cumbersome wires or battery replacements. This seemingly futuristic vision is one step closer to reality thanks to a groundbreaking advancement by researchers at MIT: the development of a completely self-powered sensor.

MIT researchers have developed a sensor that can harvest energy from its surroundings without needing a battery or a wired connection. This sensor could be used to monitor the performance and efficiency of machines in hard-to-reach places, such as inside a ship’s engine.

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The sensor is a temperature-sensing device that can clip around a wire that carries electricity, such as the one that powers a motor. The sensor can then capture the magnetic field energy generated by the current in the wire and use it to measure the motor’s temperature.

“This is ambient power — energy that I don’t have to make a specific, soldered connection to get. And that makes this sensor very easy to install,” says Steve Leeb, the senior author of the paper and a professor of electrical engineering and computer science and mechanical engineering at MIT.

The paper, featured in the January issue of the IEEE Sensors Journal, describes the design principles and challenges of creating such a battery-free, self-powered sensor.

A roadmap for energy-harvesting sensors

The researchers provide a design guide that helps engineers balance the available energy in the environment with the sensing needs of the device. The guide covers the key components of the sensor, such as the energy harvester, the energy storage, the power management, the sensing circuit, and the communication module.

The sensor can continuously sense and control the energy flow during operation and store excess energy for later use. The sensor can also cold start, meaning it can boot up its electronics with no initial voltage using a network of integrated circuits and transistors.

The design framework is not limited to sensors that use magnetic field energy but can also be applied to sensors that use other energy sources, such as vibrations or sunlight. The researchers envision that this framework could enable the development of networks of sensors for various applications, such as factories, warehouses, and commercial spaces, that are cheaper and easier to install and maintain.

“We have provided an example of a battery-less sensor that does something useful, and shown that it is a practically realizable solution. Now others will hopefully use our framework to get the ball rolling to design their sensors,” says Daniel Monagle, the lead author of the paper and a graduate student of electrical engineering and computer science at MIT.

A potential solution for ship systems

The battery-free, energy-harvesting sensor could also have implications for the monitoring of ship systems, according to John Donnal, an associate professor of weapons and controls engineering at the U.S. Naval Academy, who was not involved in the work.

Donnal says that getting power on a ship can be difficult, as there are few outlets and strict restrictions on what equipment can be plugged in. He says that measuring the vibration of a pump, for example, could provide real-time information on the health of the bearings and mounts, but powering a retrofit sensor would require too much additional infrastructure.

“Energy-harvesting systems like this could make it possible to retrofit a wide variety of diagnostic sensors on ships and significantly reduce the overall cost of maintenance,” Donnal says.

A maintenance-free system

The researchers also explain how they avoided using a battery, which would add complexity and safety risks to the sensor. Instead, they incorporated internal energy storage, including a series of capacitors. Capacitors are simpler than batteries, as they store energy in the electrical field between conductive plates. They can be made from different materials and tuned to various operating conditions, safety requirements, and available space.

The team carefully designed the capacitors so they are big enough to store the energy the device needs to turn on and start harvesting power but small enough that the charge-up phase doesn’t take too long.

They also ensured that the capacitors can hold enough energy even if some leaks out over time, as the sensor might go weeks or months before turning on to take a measurement.

“You might not even have the luxury of sending out a technician to replace a battery. Instead, our system is maintenance-free. It harvests energy and operates itself,” Monagle says.

An Energy Management Design Guide for Self-Powered Sensors.

IEEE Sensors Journal 

Smart energy management

The researchers also developed a series of control algorithms that dynamically measure and budget the energy collected, stored, and used by the device. A microcontroller, the “brain” of the energy management interface, constantly checks how much energy is stored and infers whether to turn the sensor on or off, take a measurement, or kick the harvester into a higher gear so it can gather more energy for more complex sensing needs.

They built an energy management circuit for an off-the-shelf temperature sensor using this design framework. The device harvests magnetic field energy to continually sample temperature data, which it sends to a smartphone interface using Bluetooth.

The researchers used super-low-power circuits to design the device but quickly found that these circuits have tight restrictions on how much voltage they can withstand before breaking down. Harvesting too much power could cause the device to explode.

To avoid that, the energy harvester operating system in the microcontroller automatically adjusts or reduces the harvest if the amount of stored energy becomes excessive.

They also found that communication — transmitting data gathered by the temperature sensor — was the most power-hungry operation.

“Ensuring the sensor has enough stored energy to transmit data is a constant challenge that involves careful design,” Monagle says.

Future vision

The researchers plan to explore less energy-intensive means of transmitting data, such as optics or acoustics. They also want to more rigorously model and predict how much energy might be coming into a system or how much energy a sensor might need to take measurements so a device could effectively gather even more data.

“If you only make the measurements you think you need, you may miss something really valuable. With more information, you might be able to learn something you didn’t expect about a device’s operations. Our framework lets you balance those considerations,” Leeb says.

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